1. Field of the Invention
The present invention relates to a transmission rate control method, a mobile communication system and a radio base station, which controls a transmission rate of uplink user data to be transmitted from a mobile station.
2. Description of the Related Art
In a conventional mobile communication system, when setting a dedicated channel, a radio network controller is configured to determine a transmission rate of uplink user data, in consideration of a radio resource of a radio base station, an amount of an interference in the uplink, a transmission power of a mobile station, a transmission processing performance of the mobile station, a transmission rate required for an upper application, and the like; and to notify the determined transmission rate of the uplink user data by a layer 3 (radio resource control layer) message to both of the mobile station and the radio base station.
Here, the radio network controller is provided at an upper level of the radio base station, and is an apparatus configured to control the radio base station and the mobile station.
In general, data communications often cause burst traffic compared with voice communications or TV communications. Therefore, it is preferable that a transmission rate of a channel used for the data communications is changed fast.
However, as shown in FIG. 1, a radio network controller integrally controls a plurality of radio base stations in general. Therefore, in the conventional mobile communication system, there has been a problem that it is difficult to perform fast control for changing the transmission rate of uplink user data (for example, per approximately 1 through 100 ms), due to the increase of processing load and processing delay in the radio network controller RNC.
In addition, in the conventional mobile communication system, there has been also a problem that costs for implementing an apparatus and for operating a network are substantially increased even when the fast control for changing of the transmission rate of the uplink user data can be performed.
Accordingly, in the conventional mobile communication system, when burst data transmission is performed as shown in FIG. 2A, the data are transmitted by accepting low-speed, high-delay, and low-transmission efficiency as shown in FIG. 2B, or by reserving radio resources for high-speed communications so as to accept the radio bandwidth resources in an unoccupied state and hardware resources in the radio base station Node B are wasted, as shown in FIG. 2C.
In each of FIGS. 2B and 2C, “radio resource” on the vertical axis refers to both of the radio band resource and the hardware resource mentioned above. Hereinafter, a radio band resource and a hardware resource are collectively referred to as a “radio resource”.
Therefore, the 3rd Generation Partnership Project (3GPP) and the 3rd Generation Partnership Project 2 (3GPP2), which are international standardization organizations of the third generation mobile communication system, have discussed a method for controlling radio resources at high speed in a layer-1 and a media access control (MAC) sub-layer (a layer-2), between the radio base station Node B and the mobile station UE, so as to utilize the uplink radio resources effectively. Such discussions or discussed functions will be hereinafter referred to as “Enhanced Uplink (EUL)”.
Hereinbelow, an example will be given for a control method of a radio resource (i.e., a transmission rate of uplink user data) for transmitting the uplink user data, which has been discussed in the “enhanced uplink”.
In this control method, a radio base station NodeB is configured to assign, so to each of mobile stations UE, a radio resource to be used for transmitting the uplink user data for the mobile station UE at a predetermined timing (TTI: transmission time interval).
Here, the radio base station NodeB is configured to notify, to each of the mobile stations UE by using an enhanced absolute grant channel (E-AGCH), an absolute grant (AG) corresponding to the radio resource to be assigned to each of the mobile stations UE.
The radio base station NodeB is also configured to notify, to each of the mobile stations UE, a relative grant (RG), which indicates changes (UP/DOWN/HOLD) in a transmission rate of the uplink user data corresponding to the radio resource assigned to each of the mobile stations UE. The radio base station NodeB transmits the RG by use of an enhanced-relative grant channel (E-RGCH).
Specifically, as shown in FIG. 3, the radio base station NodeB is configured to notify, to each of the mobile stations UE1 to UE3, an SG corresponding to the radio resources assigned to each of the mobile stations UE1 to UE3, by use of an AG (granted value) or an RG (UP/DOWN/HOLD) through the E-AGCH or the E-RGCH.
Note that, in each TTI (sub-frame), the radio base station NodeB is configured to send a notification of the AG or the RG only to the mobile station allowed to transmit the uplink user data through an enhanced-dedicated physical data channel (E-DPDCH).
Here, when the mobile stations UE1 to UE3 are not notified of the AG nor the RG, the mobile stations UE1 to UE3 continuously use the current SG. Note that, in a general mobile communication system, the SG assigned to the mobile stations UE1 to UE3 is set to be the same value for a period of several 10 ms to several 100 ms, in many cases.
A related technique is disclosed in Japanese Patent Publication No. 2007-517431.
However, the mobile communication system employing the “EUL” method as described above has the following problem. Detailed description will be given by referring to FIG. 4. In the mobile station UE3 of FIG. 4, transmission of uplink user data is completed in the sub-frame #2, and assigned radio resources are not used in the sub-frame #3 and #4. Thereafter, uplink user data to be transmitted is generated again in the sub-frame #5. In such case, a transmission power for transmitting an E-DPDCH from the mobile station UE is drastically increased during switching from the sub-frame #4 to the sub-frame #5. Accordingly, reception characteristics of the radio base station NodeB are deteriorated. Moreover, the dispersion of a received total wideband power (RTWP) measured at the radio base station NodeB is increased; thereby a measurement error becomes large.
Moreover, the radio resources assigned to the mobile station UE3 in the sub-frames #3 and #4 are excessive, and these radio resources are wasted since these radio resources cannot be used by the mobile stations UE1 and UE2.